Methods of making a bonded assembly and a re-entrant structure, and method of transfer printing a masking layer

ABSTRACT

A method of making a bonded polymeric assembly by transfer printing comprises contacting a stamp with a solid-phase ink comprising a photoresist to form an inked stamp, where the solid-phase ink is reversibly bound to the stamp. The inked stamp is aligned with an object comprising the photoresist and is stamped onto the object. The stamp is then removed, thereby transferring the solid-phase ink onto the object. The solid-phase ink is thermally joined with the object. Thus, a bonded polymeric assembly comprising a bonded joint between the solid-phase ink and the object is formed.

RELATED APPLICATIONS

The present patent document claims the benefit of priority under 35U.S.C. 119(e) to U.S. Provisional Patent Application No. 62/700,455,filed on Jul. 19, 2018, and hereby incorporated by reference in itsentirety.

FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT

This invention was made with government support under contract numberCMMI-1351370 awarded by the National Science Foundation. The governmenthas certain rights in the invention.

TECHNICAL FIELD

The present disclosure is related generally to microfabrication and moreparticularly to transfer printing and thermal joining to form bondedmicroassemblies.

BACKGROUND

Photolithography-based microfabrication, which includes layer by-layertarget material deposition and patterning until desired structures areformed, is ubiquitous due to improved photoresists, photolithographytechniques and etching methods. Currently, photolithographic resolutionis in the realm of tens of nanometers, enabling the fabrication ofdensely-packed, high-performing two-dimensional (2D) devices. On theother hand, further advances in microfabrication towards threedimensional (3D) heterogeneous integration are becoming more challengingdue to the layer-by-layer nature of photolithographic processing.Approaches such as multilayered photolithography and deep proton writinghave been explored to overcome the limitations of layer-by-layerprocessing; however, there has been little success in terms of broadadaptation in microfabrication.

Fabricating microsystems presents a set of challenges distinct fromthose that exist for manufacturing macroscale devices. Chief among thesechallenges is the difficulty of manipulating individual objects due tovanishing body forces compared with surface forces. Owing to thesechallenges, monolithic microfabrication, such as layer-by-layerdeposition and lithographic patterning, is commonly used to fabricatemicrosystems. However, these conventional approaches have substantialdrawbacks for the fabrication of non-planar structures. For example,complex and lengthy process steps to selectively and precisely depositand etch materials without damaging those already in place may berequired. Even with well-crafted recipes, such fabrication methods havesevere limitations in terms of producible geometries and compositions.

Modifying a surface that has a low intrinsic contact angle (γ<90°) torepel liquid droplets could be beneficial for a number of industrial andconsumer applications. The repellency of a surface can be enhanced byforming overhang or re-entrant structures that can support liquiddroplets with low surface tension. By utilizing such re-entrantstructures, the contact area between the surface and the liquid may begreatly reduced to improve the repellency and lower the adhesion ofliquids in the Cassie state. However, conventional methods to createre-entrant surfaces rely on expensive or time-consuming top-downapproaches, such as plasma etching, electrical discharge machining, andlaser cutting. In addition, typical re-entrant structures, which arebased on high-aspect-ratio protrusions, may be readily damaged underexternal forces. Therefore, a cost-effective method to fabricate robustre-entrant surfaces would be advantageous.

BRIEF SUMMARY

A method of making a bonded polymeric assembly by transfer printingcomprises contacting a stamp with a solid-phase ink comprising aphotoresist to form an inked stamp, where the solid-phase ink isreversibly bound to the stamp. The inked stamp is aligned with an objectcomprising the photoresist and is stamped onto the object. The stamp isthen removed, thereby transferring the solid-phase ink onto the object.The solid-phase ink is thermally joined with the object. Thus, a bondedpolymeric assembly comprising a bonded joint between the solid-phase inkand the object is formed.

A method of making a bonded assembly by transfer printing comprisescontacting a stamp with a solid-phase ink to form an inked stamp, wherethe solid-phase ink is reversibly bound to the stamp. The inked stamp isaligned with an object and is stamped onto the object. The stamp is thenremoved, thereby transferring the solid-phase ink onto the object. Thesolid-phase ink and the object are heated with a laser beam to effectthermal joining. Thus, a bonded assembly comprising a bonded jointbetween the solid-phase ink and the object is formed.

A method of fabricating a re-entrant structure by transfer printingcomprises contacting a stamp with a solid-phase ink comprising a polymerto form an inked stamp, where the solid-phase ink includes an array ofthrough-thickness holes and is reversibly bound to the stamp. The inkedstamp is aligned with an object comprising a polymer and is stamped ontothe object. The stamp is then removed, thereby transferring thesolid-phase ink onto the object. The solid-phase ink is oriented on theobject so as to comprise a suspended portion not supported by the objectthat includes the array of through-thickness holes. The solid-phase inkis thermally joined with the object. Thus, a re-entrant structurecomprising the suspended portion and including a bonded joint is formed.

A method of transfer printing a masking layer comprises contacting astamp with a solid-phase ink comprising a polymer to form an inkedstamp, where the solid-phase ink is reversibly bound to the stamp. Theinked stamp is aligned with a target substrate and is stamped onto thetarget substrate. The stamp is then removed, thereby transferring thesolid-phase ink comprising the polymer onto the target substrate to forma masking layer of a predetermined pattern. The target substrateincludes one or more unmasked portions not covered by the masking layer.The one or more unmasked portions of the target substrate are processed,and then the masking layer is removed.

BRIEF DESCRIPTION OF THE DRAWINGS

FIGS. 1A-1E show exemplary steps in transfer printing a solid-phase inkonto an object, followed by thermal joining to form a bonded assembly.In this example, the solid-phase ink(s) and the object(s) comprise thesame material (e.g., a photoresist) such that the bonded assembly has amonolithic structure.

FIGS. 2A and 2B show scanning electron microscope (SEM) images of abonded polymeric assembly comprising a photoresist that includes anoverhanging or suspended portion.

FIG. 3 is an SEM image of a bonded polymeric assembly comprising acantilever structure.

FIG. 4 is a schematic of a bonded polymeric assembly comprising are-entrant structure.

FIG. 5 is a schematic of a bonded assembly having a composite structure,where the solid-phase ink(s) and the object(s) comprise differentmaterials.

FIGS. 6A and 6B are SEM images of fabricated re-entrant structures.

FIGS. 7A and 7B show, respectively, a water droplet on a re-entrantsurface comprising a photoresist, fabricated as described in thisdisclosure, and a water droplet on a flat surface comprising the samephotoresist.

FIGS. 8A-8D are schematics illustrating fabrication of solid-phase inkscomprising a photoresist such as SU-8.

FIGS. 9A-9F are schematics illustrating transfer printing of solid-phaseinks comprising a photoresist such as SU-8.

FIGS. 10A and 10B show steps in fabricating the re-entrant structure ofFIGS. 6A and 6B, where in FIG. 10A an exemplary photoresist (SU-8) layeris photolithographically patterned on a silicon substrate; and in FIG.10B a SU-8 ink with a circular hole array is separately patterned on adonor substrate and assembled on the patterned SU-8 layer.

DETAILED DESCRIPTION

A microassembly technique—termed “micro-Lego”—which involves transferprinting of individual microscale building blocks (“solid-phase inks”)followed by thermal joining without any intermediate adhesive has beendeveloped for 3D microfabrication. The microassembly method typicallyutilizes a microstructured polymeric stamp to achieve a high adhesionon/off ratio, which facilitates printing of solid-phase inks ontovirtually any surface without additional adhesive layers. After transferprinting, the solid-phase inks may be permanently joined via thermalprocessing. As demonstrated in this disclosure, the capability ofmicro-Lego can be extended to construct practical 3D objects anddevices, such as robust cantilever structures and re-entrant surfacescapable of repelling liquid droplets. In addition, the microassemblytechnique may be used to assemble and join photoresist-based solid-phaseinks to obtain structures not achievable with conventionalphotolithographic processing, such as the cantilever and re-entrantstructures mentioned above and masking layers on nonuniform surfaces.Finally, a rapid, laser-based approach may be employed for thermaljoining, rendering the microassembly method capable of high-throughputadditive manufacturing.

FIGS. 1A-1E show an exemplary method to prepare a bonded polymericassembly by transfer printing and thermal joining of solid-phase inks.FIG. 1A illustrates several solid-phase inks 102 of arbitrarily chosenshapes supported by a donor substrate 104 and ready for microassembly.In this example, each of the solid-phase inks 102 comprises aphotoresist. Preparation of the solid-phase inks 102 for easy retrievalfrom the donor substrate 104 takes place prior to transfer printing, asdescribed below.

Referring to FIGS. 1B and 1C, the method comprises contacting a stamp106 with a solid-phase ink 102 comprising a photoresist to form an inkedstamp 108. The stamp 106 is capable of reversible binding of thesolid-phase ink 102. The inked stamp 108 (i.e., the stamp 106 with thesolid-phase ink 102 reversibly bound to the stamp 106) is removed fromthe donor substrate 104 and aligned with an object 112, which may bedisposed on a receiving substrate 110. The motion of the stamp 106 maybe controlled manually or automatically, such as with a motioncontroller. The object 112 may be a previously deposited or printedsolid-phase ink 102, or the object 112 may be the receiving substrate110. In this example, the object 112 also comprises the photoresist. Asshown in FIG. 1D, the inked stamp 108 is stamped onto the object 112.Upon removal of the stamp 106, the solid-phase ink 102 remains on theobject 112, thereby completing transfer printing of the solid-phase ink102.

Referring to FIG. 1E, during or after transfer printing, the solid-phaseink 102 is thermally joined with the object 112. In other words, beforeor after removal of the stamp 106, the solid-phase ink 102 and theobject 112 are heated to effect bonding at an interface therebetween,forming a bonded joint 118. In the example of FIG. 1E, thermal joiningof the solid-phase ink 102 and the object 112 is effected by laserheating, typically after removing the stamp 106.

Generally speaking, thermal joining may occur when the interface betweenthe solid-phase ink 102 and the object 112, each comprising aphotoresist, reaches a temperature sufficient for bonding, such as atemperature in a range from about 120° C. to about 180° C. Thermaljoining may be carried out using conductive, convective and/or radiantheating. For example, thermal joining may be carried out in a furnace orwhile the receiving substrate is adjacent to or on a heat source, suchas a hot plate. Alternatively, as described above, the heating may belocalized by employing a laser beam or a heat gun. Besides localization,an advantage of laser heating is speed. Using a hot plate or furnace,thermal joining may be effected within a period of minutes (e.g., from 1min to 60 min, or more typically from 10 min to about 30 min). Incontrast, using a laser beam, such as a laser beam generated by a pulsedlaser, thermal joining may occur almost instantaneously (e.g., in a timeperiod of less than one second, such as 500 ms or less, or 1 ms orless). Due to the pulse duration of commercially available pulsedlasers, the time period for thermal joining is typically at least 1 ps,or at least 1 ns. In response to the laser pulse, the solid-phase ink102 and the object 112 may experience an extremely rapid rise and fallin temperature, yielding a strong interfacial bond (bonded joint 118).It is known that the photoresist SU-8 strongly absorbs light having awavelength around 300 nm. Accordingly, it may be beneficial to employ alaser wavelength in a range from about 200 nm to about 400 nm for thelaser heating. Advantageously, no external pressure is required duringthermal joining.

Thus, a bonded polymeric assembly 100 including a bonded joint 118between the solid-phase ink 102 and the object 112 may be formed.Preferably, the bonded joint may exhibit an interfacial joining strengthof at least about 0.9 J/m². Like the solid-phase ink 102 and the object112, the bonded polymeric assembly 100 may comprise a photoresist. Thephotoresist may be an epoxy-based photoresist. Since solidification orcrosslinking of the photoresist may be carried out during fabrication ofthe solid-phase ink 102 (prior to transfer printing and joining to formthe bonded assembly 100), the photoresist may be described as acrosslinked photoresist. Examples of suitable photoresists may includenegative or positive photoresists, such as SU-8, KMPR, UVN™, ma-N,and/or polyimide. The joining strength of a thermally joined SU-8interface has been examined experimentally through nanoindentation testsand numerical analysis, as described in the Examples below.

The method may further include repeating the contacting, aligning,stamping, removing, and (optionally) the thermal joining to depositadditional solid-phase ink(s) and form a bond with the underlyingobject, which may be a previously deposited solid-phase ink. It isunderstood that one sequence of contacting, aligning, stamping, andremoving constitutes a single transfer printing step. In one approach,thermal joining may be carried out only after multiple transfer printingsteps to attain the desired arrangement of solid-phase inks. In anotherapproach, thermal joining may be carried out multiple times, e.g., aftereach sequence of contacting, aligning, stamping, and removing. Thebonded polymeric assembly 100 may include at least two of thesolid-phase inks 102 stacked on a receiving substrate 110, as shownschematically in FIG. 1E.

Depending on the arrangement of the solid-phase inks 102, the bondedpolymeric assembly 100 may include an overhanging or suspended portionor another 3D geometry that is difficult or impossible to fabricateusing conventional photolithographic patterning. FIGS. 2A and 2B arescanning electron microscope (SEM) images of 100 μm×100 μm and 100μm×300 μm SU-8 inks that have been transfer printed and thermallyjoined. Each of the resulting bonded polymeric assemblies 100 comprisesa monolithic structure 200 a,200 b that includes at least one suspendedportion 214. As would be recognized by the skilled artisan, the term“suspended portion” refers to a part of the bonded polymeric assemblythat is physically unsupported by another structure (e.g., an object, apreviously deposited ink, the receiving substrate). As described ingreater detail in the examples below, the bonded polymeric assembly 100may comprise a cantilever structure 300 with an extended suspendedportion 314, as shown for example in FIG. 3. In another example, asshown in FIG. 4, the bonded polymeric assembly 100 may comprise are-entrant structure 400, where the central suspended portion 414provides a low wettability surface for a liquid droplet. In thisexample, upon removal of the stamp and transfer of the solid-phase ink102 to the object 112, the solid-phase ink 102 may be oriented on theobject 112 so as to include a suspended portion 414 not supported by theobject 112 that includes an array of through-thickness holes 416. Uponthermally joining the solid-phase ink 102 with the object 112, are-entrant structure 400 comprising the suspended portion 414 andincluding a bonded joint 118 may be formed.

The stamp employed for transfer printing may include a polymericmaterial comprising a shape memory polymer or an elastomer, such aspolydimethylsiloxane (PDMS). Suitable stamps are described in, forexample, U.S. Pat. No. 9,412,727, issued Aug. 9, 2016, and U.S. Pat. No.10,046,353, issued Aug. 14, 2018, both of which are hereby incorporatedby reference in their entirety.

In another embodiment of the method, a bonded assembly may be fabricatedby transfer printing utilizing a solid-phase ink and an objectcomprising the same or different materials (which may or may notcomprise a photoresist), followed by thermal joining carried out withlaser heating to facilitate rapid fabrication. As described above inreference to FIGS. 1B and 1C, a stamp 106 is contacted with asolid-phase ink 102 to form an inked stamp 108, where the solid-phaseink 102 is reversibly bound to the stamp 106. In this embodiment,however, the solid-phase ink 102 may comprise any material, such as apolymer, metal or alloy, ceramic, or semiconductor. The inked stamp 108(i.e., the stamp 106 with the solid-phase ink 102 reversibly bound tothe stamp 106) is removed from the donor substrate and aligned with anobject 112 for transfer printing. The object 112 may comprise anymaterial, such as a polymer, metal or alloy, ceramic, or semiconductor,which may be the same as or different from that of the solid-phase ink102. The object 112 may be a previously deposited solid-phase ink 102 ora receiving substrate 110. As shown in FIG. 1D, the inked stamp 108 isstamped onto the object 112. Upon removal of the stamp 106, thesolid-phase ink 102 remains on the object 112, thereby completingtransfer printing.

Referring to FIG. 1E, during or after transfer printing, the solid-phaseink 102 is thermally joined with the object 112. In other words, beforeor after removal of the stamp 106, the solid-phase ink 102 and theobject 112 are heated to effect bonding at an interface therebetween,forming a bonded joint 118. As illustrated in FIG. 1E, thermal joiningof the solid-phase ink 102 and the object 112 is effected by laserheating, typically after removing the stamp 106. Laser heating may becarried out with a laser beam, which may be generated by a pulsed laser.Generally speaking, thermal joining may occur when the interface betweenthe solid-phase ink 102 and the object 112 reaches a temperaturesufficient for bonding. In the case of a polymer, such as a photoresist,the temperature may be in the range from about 120° C. to about 180° C.With laser heating, as described above, thermal joining may occur almostinstantaneously, such as in a time period of less than one second (e.g.,500 ms or less, or 1 ms or less). Due to the pulse duration ofcommercially available pulsed lasers, the time period for thermaljoining is typically at least 1 ps, or at least 1 ns. In response to thelaser pulse, the solid-phase ink 102 and the object 112 may experiencean extremely rapid rise and fall in temperature, yielding a stronginterfacial bond (bonded joint 118). A laser wavelength which isstrongly absorbed by the material(s) of the solid-phase ink and theobject may be selected for laser heating. Advantageously, as describedabove, no external pressure is required during thermal joining.

Thus, a bonded assembly 100 including a bonded joint 118 between thesolid-phase ink 102 and the object 112 may be formed. If the solid-phaseink 102 and the object 112 comprise the same material, then the bondedassembly 100 may be a monolithic structure comprising the material, asdescribed above. For example, if the ink 102 and the object 112 comprisea polymer, a bonded polymeric assembly 100 having a monolithic structure200 a,200 b comprising the polymer may be formed upon bonding, as shownfor example in FIGS. 2A and 2B. If the solid-phase ink 102 and theobject 112 comprise different materials, then the bonded assembly 100may be a composite structure 500 comprising the different materials, asshown for example in FIG. 5. Advantageously, in either case, the bondedjoint may exhibit an interfacial joining strength of at least about 0.9J/m².

The method may further include repeating the contacting, aligning,stamping, removing, and (optionally) the thermal joining to depositadditional solid-phase ink(s) and form a bond with the underlyingobject, which may be a previously deposited solid-phase ink. As setforth above, one sequence of contacting, aligning, stamping, andremoving constitutes a single transfer printing step. In one approach,thermal joining may be carried out only after multiple transfer printingsteps to attain the desired arrangement of solid-phase inks. In anotherapproach, thermal joining may be carried out multiple times, e.g., aftereach sequence of contacting, aligning, stamping, and removing.

The bonded assembly 100 may include at least two of the inks 102 stackedon a receiving substrate 110. As indicated above, the bonded assemblymay be a monolithic structure comprising a single material or compositestructure comprising multiple (different) materials. Depending on thearrangement of the solid-phase inks 102, the bonded assembly may includean overhanging or suspended portion or another 3D geometry that isdifficult or impossible to fabricate using conventionalphotolithographic patterning. As described above, the bonded assembly100 may comprise a cantilever structure 300 or a re-entrant structure400 for repelling liquid droplets, as shown in FIGS. 3 and 4,respectively.

The re-entrant structure 400, which comprises a first object disposed onand bonded to a second object, where each of the first and secondobjects comprises a polymer, may be fabricated according to eitherembodiment of the method described above. The first object is orientedon the second object so as to include a suspended portion not supportedby the second object, and the suspended portion includes an array ofthrough-thickness holes, as illustrated in FIGS. 4, 6A and 6B. The firstand second objects may comprise solid-phase inks 102 transfer printedonto a substrate 110, followed by thermal joining (e.g., carried out bylaser heating) to form a bonded joint 118 between the first and secondobjects. As can be seen in FIGS. 6A and 6B, the first object (“SU-8ink”) may include a solid border extending around all or part of thearray of through-thickness holes, and the bonded joint may comprise thesolid border of the first object and an underlying portion of the secondobject. The bonded joint between the first and second objects (labeled“SU-8 ink” and “Patterned SU-8”) may have an interfacial joiningstrength of at least about 0.9 J/m².

When exposed to a liquid droplet, the re-entrant structure 400 may repelthe liquid droplet with an apparent contact angle of at least about 90°,and in some cases the apparent contact angle may be at least about 105°,as shown for example in FIG. 7A for an exemplary SU-8 re-entrantstructure holding a water droplet. For comparison, a water droplet on aSU-8 flat surface with a much smaller apparent contact angle is shown inFIG. 7B. Measurement of the apparent contact angle is illustrated in thefigures and is discussed theoretically below.

In the example of FIGS. 6A and 6B, the polymer comprises a photoresist,specifically, SU-8. Generally speaking, the photoresist may be selectedfrom: SU-8, KMPR, UVN™, ma-N, and polyimide. The first object maycomprise the same or a different polymer as the second object. Thepolymer may comprise an intrinsic contact angle of at least about 90°;in other words, the polymer may be hydrophobic. Alternatively, thepolymer may comprise an intrinsic contact angle of less than 90°; inother words, the polymer may be hydrophilic. The intrinsic contact anglemay be less than 80°. Accordingly, the apparent contact angle may be atleast about 25% larger than an intrinsic contact angle of the polymer.

A method of fabricating the re-entrant structure may comprise contactinga stamp with a solid-phase ink comprising a polymer to form an inkedstamp, where the solid-phase ink includes an array of through-thicknessholes and is reversibly bound to the stamp. The inked stamp may bealigned with an object comprising a polymer, and the inked stamp may bestamped onto the object. When the stamp is removed, the solid-phase inkmay be transferred onto the object, and the solid-phase ink may beoriented on the object so as to include a suspended portion notsupported by the object, where the suspended portion includes the arrayof through-thickness holes. The solid-phase ink may be thermally joinedwith the object, thereby forming a re-entrant structure comprising thesuspended portion and including a bonded joint. The thermal joining maybe carried out using any approach described herein, such as laserheating. The stamp, re-entrant structure, polymer, solid-phase ink,object and bonded joint may have any of the characteristics describedabove or elsewhere in this disclosure.

Also described in this disclosure is a method of transfer printing amasking layer, which may be understood in reference to FIGS. 9A-9F. Themethod comprises contacting a stamp 106 with a solid-phase ink 102comprising a polymer to form an inked stamp 108, where the solid-phaseink 102 is reversibly bound to the stamp 106, and aligning the inkedstamp 108 with a target substrate 110. The target substrate 110 maycomprise a semiconductor wafer. As described above, the polymer may be aphotoresist such as SU-8, KMPR, UVN™, ma-N, and/or polyimide. The inkedstamp 108 is stamped onto the target substrate 110, and the stamp 106 isremoved, thereby transferring the solid-phase ink 102 comprising thepolymer onto the target substrate 110 to form a masking layer 120 of apredetermined pattern on the target substrate 110. The method mayinclude multiple steps of transferring the solid-phase ink onto thetarget substrate 110, such that the contacting, aligning, stamping, andremoving are repeated in order to form the masking layer 120. Thermaljoining using a localized heat source, such as a laser beam as describedabove, may allow the solid-phase inks 102 to be bonded to each other toform a robust masking layer 120 of the desired pattern without bondingthe masking layer 120 to the target substrate 110.

The target substrate 110 includes one or more unmasked portions 122 notcovered by the masking layer 120. The target substrate 110 may includeone or more trenches, bumps or other surface features that inhibitspin-casting of photoresist. The one or more unmasked portions 122 ofthe target substrate 110 may undergo processing, and, after theprocessing, the masking layer 120 may be removed. The processing of theone or more unmasked portions 122 of the target substrate 110 mayinclude material deposition and/or etching. The removal of the maskinglayer 120 may take place using one of a number of different approaches.In one example, the removal may entail contacting the masking layer withthe stamp, such that the masking layer is reversibly bound to the stamp,and the stamp may be withdrawn to remove the masking layer from thetarget substrate. Using this approach, a portion or an entirety of themasking layer may be removed in a single step. In another example,removing the masking layer may comprise applying a removal liquid, suchas developer or acetone, to the target substrate. In yet anotherexample, the masking layer may be removed by employing apressure-sensitive adhesive to peel off the masking layer.

EXAMPLES Fabrication of SU-8 Solid-Phase Inks

The process may begin with preparation of a donor substrate, as shownschematically in FIGS. 8A-8D. The donor substrate includes in thisexample an array of microscale SU-8 inks. The solid-phase inks exhibitnegligible adhesion with the donor substrate to facilitate easyretrieval. Referring to FIG. 8A, polymethylmethacrylate (PMMA) is spincoated at 3000 rpm on a Si substrate and cured at 150° C. for 1 min.Subsequently, SU-8 50 is coated onto the PMMA at 3000 rpm, as shown inFIG. 8B, and photolithographically patterned, as shown in FIG. 8C, andhard baked (heated) at 110° C. for 1 h. Here, sufficient baking canfully cure (crosslink) the SU-8 inks and terminate epoxy chains that areutilized in micro-Lego for permanent joining, while insufficient bakingmay leave the SU-8 inks prone to damage during the PMMA removal step.The fully cured or crosslinked SU-8 inks are submerged in an acetonebath to remove the PMMA, which engenders the SU-8 inks merely placed onthe Si substrate with weak surface interaction, as illustrated in FIG.8D.

Transfer Printing and Joining of SU-8 Inks

Referring again to the schematics of FIGS. 9A and 9B, the prepared SU-8ink 102 may be brought into contact with a polydimethylsiloxane (PDMS)stamp that features multiple microtips for an enhanced adhesion on/offratio. Rapid retrieval of the stamp 106 after a high preloading allowsfor reversible binding of the SU-8 ink, as illustrated in FIG. 9C. Theinked stamp 108 may then delivered to a target location on a Sireceiving substrate 110 and brought into contact with the printingsurface, as shown in FIG. 9D. The stamp 106 may be subsequentlyretracted slowly, as shown in FIG. 9E, which results in release andprinting of the SU-8 ink 102 onto the target location, as illustrated inFIG. 9F. Since the printed SU-8 ink 102 typically adheres to the surface110 underneath by weak van der Waal's interaction, delicate handling maybe advisable prior to permanent joining. Transfer printing is repeatedwith additional SU-8 ink(s) until a desired structure is constructed,and may be followed by thermal joining. In one experimental example,thermal processing takes place at 150° C. for 1 h utilizing a hot platedisposed underneath the Si receiving substrate.

Fabrication of Cantilever Structure

Transfer printing and joining is employed to form a SU-8 cantilever beamspecimen for nanoindenter tests. The material of choice for SU-8 inks isSU-8 50, which yields—40 μm thickness (t) film upon 3000 rpm spincoating. A 100 μm by 100 μm square ink is first transfer printed onto aSi substrate and a 100 μm by 300 μm rectangular ink is transfer printedwith precise alignment such that a 200 μm long beam is suspended on a100 μm by 100 μm spacer. The transfer printed SU-8 inks are joined at150° C. to finalize the construction of the cantilever specimen as shownin FIG. 3.

The mechanical joining strength at the joining interface may bequantified by investigating the energy release rate, which refers to theamount of energy required for a crack to grow a unit length based onlinear elastic fracture mechanics, as described in U.S. ProvisionalPatent Application No. 62/700,455, the priority application for thisdisclosure, which was incorporated by reference above. Adopting thetensile stress from finite element analysis in conjunction with 1 μm ofcrack tip length, an energy release rate of 0.992 J/m² (about 1 J/m²) iscalculated.

Fabrication of Re-entrant Structure

In this example, the fabrication process of a SU-8 re-entrant structure(or re-entrant surface) starts with photolithographic patterning of aSU-8 layer on a Si substrate. SU-8 50 is spin coated at 3000 rpm toobtain a 40 μm thick SU-8 layer. Following exposure with a photomask anddevelopment, a 4 mm square trench pattern that is used as cornerstone ofthe SU-8 re-entrant structure is constructed, as shown in FIG. 10A. A 6mm square SU-8 ink with an array of circular holes of 125 μm in diameteris separately fabricated as described above. Subsequently, micro-Lego isconducted to assemble the prepared SU-8 ink on the patterned SU-8 layer,as illustrated in FIG. 10B. Thermal processing to join the patternedSU-8 layer and the transfer printed SU-8 ink on a hot plate at 150° C.completes the fabrication of a SU-8 re-entrant structure. FIGS. 6A and6B show SEM images of the assembled SU-8 re-entrant structure. There isan approximately 40 μm gap between the assembled SU-8 ink and the Sisurface, which enables the SU-8 re-entrant structure to repel a liquiddroplet, as illustrated in FIG. 4. Wetting characteristics of the SU-8re-entrant surface are examined using a goniometer after placing a 3 μlwater droplet on it.

In order for a surface with a low intrinsic contact angle (Y<90°) torepel a liquid droplet, the surface may require features—a re-entranttopology—to suspend the droplet. The re-entrant topology can suspend theliquid due to the formation of upward surface tension (Υ) over theliquid meniscus as indicated in the inset image of FIG. 4. On there-entrant structure, a base of droplet contacts the solid and gassimultaneously. If the liquid-solid contact fraction (f) is smallenough, the apparent contact angle of a liquid droplet (θ*) becomeslarger than 90° and the surface starts to repel the liquid. Assuming anideally flat liquid-solid contact surface and negligible meniscuscurvature, the apparent contact angle θ* of a suspended droplet can becalculated by Cassie-Baxter equation as below:cos θ*=f(cos θ_(Y)+1)−1

According to the SU-8 re-entrant surface design in this example, f is0.56 and the Y of the flat SU-8 surface is measured as 73±5° (FIG. 7B).From the above equation, the theoretical θ* can be calculated as 106°which is similar to the measured θ0* on the SU-8 re-entrant surface(118±5°) (FIG. 7A). The disparity between theoretical and experimentalvalues can be explained by the relatively thick sidewall of the holes inthe SU-8 ink compared to its circular hole diameter. The thick sidewallcauses enlarged liquid-solid contact area as well as droplet pinningaround circular holes. Conventional re-entrant surfaces are composed ofa protruded pillar array with a high aspect ratio, which inherently isfragile under external abrasion or forces. In this context, thedemonstrated SU-8 re-entrant structure is fabricated from a singlecontinuous single film that is more robust to external perturbations.

Although the present invention has been described in considerable detailwith reference to certain embodiments thereof, other embodiments arepossible without departing from the present invention. The spirit andscope of the appended claims should not be limited, therefore, to thedescription of the preferred embodiments contained herein. Allembodiments that come within the meaning of the claims, either literallyor by equivalence, are intended to be embraced therein.

Furthermore, the advantages described above are not necessarily the onlyadvantages of the invention, and it is not necessarily expected that allof the described advantages will be achieved with every embodiment ofthe invention.

The invention claimed is:
 1. A method of making a bonded polymericassembly by transfer printing, the method comprising: contacting a stampwith a solid-phase ink comprising a crosslinked photoresist to form aninked stamp, the solid-phase ink being reversibly bound to the stamp;aligning the inked stamp with an object comprising the crosslinkedphotoresist; stamping the inked stamp onto the object; removing thestamp, thereby transferring the solid-phase ink onto the object; andthermally joining the solid-phase ink with the object, thereby forming abonded polymeric assembly including a bonded joint between thesolid-phase ink and the object.
 2. The method of claim 1, wherein thebonded joint has an interfacial joining strength of at least about 0.9J/m².
 3. The method of claim 1, wherein the solid-phase ink is orientedon the object so as to include a suspended portion not supported by theobject.
 4. The method of claim 1, wherein the object comprises apreviously deposited solid-phase ink deposited prior to the stamping. 5.The method of claim 1, wherein the thermal joining comprises heating thesolid-phase ink and the object with a laser beam, a heat gun, a hotplate, and/or a furnace.
 6. The method of claim 5, wherein the laserbeam has a wavelength in range from about 200 nm to about 400 nm.
 7. Themethod of claim 5, wherein the laser beam is generated by a pulsedlaser.
 8. The method of claim 1, wherein no external pressure is appliedduring the thermal joining.
 9. A method of making a bonded assembly bytransfer printing, the method comprising: coating a photoresist onto adonor substrate; photolithographically patterning the photoresist tofabricate a solid-phase ink comprising the photoresist on the donorsubstrate; contacting a stamp with the solid-phase ink to form an inkedstamp, the solid-phase ink being reversibly bound to the stamp; removingthe inked stamp from the donor substrate; aligning the inked stamp withan object; stamping the inked stamp onto the object; removing the stamp,thereby transferring the solid-phase ink onto the object; and heatingthe solid-phase ink and the object with a laser beam to effect thermaljoining, thereby forming a bonded joint between the solid-phase ink andthe object.
 10. The method of claim 9, wherein the laser beam isgenerated by a pulsed laser.
 11. The method of claim 9, wherein thermaljoining to form the bonded joint occurs in less than one second.
 12. Amethod of transfer printing a masking layer, the method comprising:contacting a stamp with a solid-phase ink comprising a crosslinkedphotoresist to form an inked stamp, the solid-phase ink being reversiblybound to the stamp; aligning the inked stamp with a target substrate;stamping the inked stamp onto the target substrate; removing the stamp,thereby transferring the solid-phase ink comprising the crosslinkedphotoresist onto the target substrate to form a masking layer of apredetermined pattern thereon, the target substrate including one ormore unmasked portions not covered by the masking layer; processing theone or more unmasked portions of the target substrate; and after theprocessing, removing the masking layer.
 13. The method of claim 12,wherein the target substrate comprises a semiconductor wafer, whereinprocessing the one or more unmasked portions of the target substratecomprises material deposition and/or etching, and wherein the targetsubstrate includes one or more trenches, bumps or other surface featuresthat inhibit spin-casting of photoresist.
 14. The method of claim 12,wherein forming the masking layer comprises multiple steps oftransferring the solid-phase ink onto the target substrate, such thatthe contacting, aligning, stamping, and removing are repeated to formthe masking layer, and wherein the solid-phase inks are thermally joinedto each other by laser heating.